The invention relates to xerographic process control, and more particularly, to the improvement for the use of existing sensors to monitor and automatically set up interdocument zone patches.
In copying or printing systems, such as a xerographic copier, laser printer, or ink-jet printer, a common technique for monitoring the quality of prints is to artificially create a "test patch" of a predetermined desired density. The actual density of the printing material (toner or ink) in the test patch can then be optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
In the case of xerographic devices, such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge-retentive surface or photoreceptor, on which the electrostatic latent image is formed and subsequently, developed by causing toner particles to adhere to areas thereof that are charged in a particular way. In such a case, the optical device for determining the density of toner on the test patch, which is often referred to as a "densitometer", is disposed along the path of the photoreceptor, directly downstream of the development of the development unit. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to charge or discharge as necessary the surface at the location to a predetermined extent.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere to the test patch electrostatically. The denser the toner on the test patch, the darker the test patch will appear in optical testing. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor, and the light absorption of the test patch is tested; the more light that is absorbed by the test patch, the denser the toner on the test patch.
In any printing system using test patches for monitoring print quality, a design problem inevitably arises of where to place these test patches, particularly on photoreceptor belts or drums. Xerographic test patches are traditionally printed in the interdocument zones on the photoreceptor. They are used to measure the deposition of toner on paper to measure and control the tone reproduction curve (TRC). Generally each patch is about an inch square that is printed as a uniform solid half tone or background area. This practice enables the sensor to read one value on the tone reproduction curve for each test patch. However, that is insufficient to complete the measurement of the entire curve at reasonable intervals, especially in a multi-color print engine. To have an adequate number of points on the curve, multiple test patches have to be created. Thus, the traditional method of process controls involves scheduling solid area, uniform halftones or background in a test patch. Some of the high quality printers contain many test patches. During the print run, each test patch is scheduled to have single halftone that would represent a single byte value on the tone reproduction curve.
Various prior art techniques have been proposed to improve the use of test patches for xerographic control. For example, pending application Ser. No. 08/527,616 filed Sep. 13, 1995 discloses a method of development control by storing a reference tone reproduction curve and providing a single test pattern including a scale of pixel values in the interdocument zone of the imagining surface. The system senses the test pattern along the scale of pixel values in the interdocument zone and responds to the sensing of the test pattern and the reference tone reproduction curve to adjust the machine operation for print quality correction. It is also known in the prior art, for example, U.S. Pat. No. 4,341,461 to image multiple test targets in the interdocument zones of the photoreceptor. For example, two test targets each having two test patches are selectively exposed singly or in overlapping relationship to provide test data to control toner dispensing and developer bias.
A difficulty with the prior art systems, however, is the timing of the placement of test patches in the interdocument zone and timing of the subsequent sensing of the test patch. Correct timing is necessary for correct readings and often differences in the inherent operation of machines and the placement of sensors in the machine results in error in the readings. Also, since timing relationships are often manually set up by service representatives, inconsistency and further error can be introduced into the machine xerographic control.
It would be desirable, therefore, to be able to improve the timing in generating and reading test patches as well as to improve timing without additional costly components. It would also be desirable to improve accuracy, eliminate machine to machine differences, and reduce set up time for a machine control.
It is an object of the present invention therefore to provide a new and improved technique for process control, in particular, for automatically timing the generation of test patches using existing machine sensors. It is another object of the present invention to sense a test pattern in a timing relationship to the edges of a sensor field of view and locate the sensor field of view with respect to the test pattern.
Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.